1. Field of the Invention
This invention relates to heating, venting, and air conditioning (HVAC) and refrigeration systems, and more particularly, to HVAC and refrigerating systems having a device for controlling the flow of fluids.
2. Description pf Related Art
This invention relates to HVAC and refrigeration systems, and in particular to HVAC refrigerating systems having a device for maximizing the efficiency of working fluids. While the invention is described in detail with respect to a conventional refrigeration or HVAC system, those skilled in the art will recognize the wider applicability of the invention disclosed hereinafter. The invention may find application with other refrigeration systems where system efficiency may be improved by monitoring specific parameters affecting that efficiency.
The operational features of conventional refrigeration systems are well known in the art. An example of such a system is a refrigerated container, such as a supermarket display case. In general, the refrigeration system includes a compressor that forces a particular working fluid, such as a refrigerant, used in the system through a condenser where the refrigerant vapor liquefies. The liquid refrigerant passes through a thermostatic expansion valve, expanding the high pressure liquid refrigerant to a low pressure vapor. The low pressure, low temperature refrigerant discharged from the thermostatic expansion valve is then directed through an evaporator for absorbing heat and thus refrigerating the space inside the container surrounding the evaporator.
In conventional refrigeration systems, the condenser is placed in an outdoor setting. It is generally known that decreases in ambient temperature at the condenser cause a proportional decrease in pressure of the refrigerant flowing through the condenser. Thus, a change in outdoor ambient temperature affects the performance of a refrigeration system.
It is desirable to operate a refrigeration system with a minimum condensing pressure. It is well-known that the efficiency of refrigeration system compressors is increased as the condensing pressure in the refrigeration system drops. Therefore, the condensing pressure should operate at an optimal minimum for a given refrigeration system design.
This optimal minimum pressure depends on various factors. For example, it is known that physical characteristics of the compressors require that a higher minimum be maintained than is absolutely required to avoid damage to the compressor itself. Excessively low compression ratios can damage internal components of reciprocating compressors or disable screw compressors due to low oil flow, for example. Further, because of the ambient temperature changes described above, this minimum should be dictated not only by the fixed components of a given refrigeration system, but also by the variable ambient conditions.
To overcome this problem with variable ambient temperatures, it is known to place a mechanical valve immediately downstream of the condenser. This mechanical valve acts to restrict the flow of the refrigerant out of the condenser, thus increasing the pressure of the refrigerant in the condenser. This valve has traditionally been a manually adjustable, mechanical valve with a fixed pressure setting. In this way, depending on the ambient weather conditions, it is possible to partially compensate condenser pressure for changes in ambient temperature.
However, adjusting the mechanical valve is a time-consuming, manual process. Because these mechanical valves are adjusted manually, the mechanical valves are generally not adjusted often, and are definitely not controlled in real time. The lack of real time control thus causes a decrease in the efficiency of the compressor.
Further, it is known that by restricting the flow of the refrigerant through the mechanical valve, the effective heat transfer surface of the condenser is reduced and, in conjunction with varying the fluid flow, results in elevating the system condensing pressure. Thus, it is desirable to actively control the condensing pressure in real time so that compressor efficiency will increase, among other things.
Other problems result from failing to actively control the condenser pressure based on changes in ambient conditions at the condenser. When the ambient temperature exceeds the saturated gas pressure setting of the mechanical valve, poor quality liquid often will result prior to the refrigerant entering the expansion valve. It is generally known that poor quality liquid can lead to a variety of operational problems including the following: improper temperatures at the evaporator and display, poor expansion valve feeding, inadequate sub-cooler capacities, and vapor lock in horizontal line runs.
Improper temperature at the display can be especially troubling in commercial refrigeration systems. In many of such refrigeration system implementations, finer temperature control is desirable. With the example grocery store case refrigeration system, several factors fuel the need for finer case temperature control. Government regulations may require more stringent temperature regulation, and requirements for longer product shelf life and improved product quality further make tighter control of case temperature a necessity. Moreover, if the ambient temperature changes, the process of manually adjusting the mechanical valve to adjust condenser pressure must be repeated.
Another problem exists with using the mechanical valve to adjust condenser pressure for changes in ambient temperature: these systems will not work properly with the gas defrost process. Gas defrost methods divert high pressure superheated or saturated gas from the compressor discharge or the receiver respectively to the evaporator that has ice formed on it. As the gaseous refrigerant condenses, the rejected heat melts the unwanted ice. Gas defrost is generally known to be the most efficient method of defrosting low temperature display cases. However, when the mechanical valve is set for optimum compressor energy efficiency, the refrigeration system is not capable of properly defrosting the evaporators. This lack of energy efficiency in the gas defrost process negates any energy efficiency improvements by utilizing the mechanical valve.
It is therefore desirable to control condenser pressure in real time such that it can be kept to a workable minimum thereby improving compressor efficiency. It is also desirable to control this condenser pressure in a way such that the gas defrost process can be utilized without decreasing the efficiency of the refrigeration system. Further, it is desirable to control condenser pressure while supplying high quality liquid refrigerant to the expansion value.
The present invention addresses these, and other, shortcomings associated with the prior art.